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How Does the Heater in TEGAM Sensors Work?

Posted: Thu Mar 14, 2013 12:02 pm
by abrush1
We get questions about how the mount heater system works, how it improves reading stability, and how a user can really know if the mount is "Ready". I think the best way to illustrate an answer is to show a graph of how the mount DC power changes as mount heater voltage and current change.
The graph is of the data from an M1130A connected to an 1830A meter when "cold" (at room temperature). The red line presents the voltage sourced by the heater controller, on the left vertical axis. The purple line shows current drawn by the heater in the mount, in 1/10ths of an Ampere, on the right axis. The blue line shows the change in DC substituted power of the thermistors, in milliwatts. The horizontal axis is in samples, one sample taken every 10 seconds.

At first, the mount temperature is too low, so the heater controller in the 1830A supplies full voltage, about 14 volts, which results in maximum heating. The substituted DC is seen to change quickly as the sensor's body warms up to operating temperature. The current drawn drops slowly as the mount warms up. This happens as the temperature-sensitive parts of the heater coil increase in resistance, allowing the heater controller to sense temperature.

The action of the temperature controller is seen between samples 400 and 500. The mount has reached design temperature, and the controller "backs off" of the drive voltage to start to maintain balance between the main heater coil and the temperature sensitive part of the heater coil. Heater current drops proportionally. Also at this point, the substituted DC power stops changing rapidly, indicating that the "zero" reading of this mount is now relatively stable. After the heater starts controlling, the temperature gradient in the mount takes another hour or so the completely stabilize, resulting in a final four or five microwatts of drift. The mount can be successfully used for calibration during that time, by taking frequent power-off readings to remove the zero drift. Taking power-off readings is always good practice. Note that the level of current depends on the ambient temperature, the construction of the mount, and the temperature that the mount is adjusted for. These variables mean that under some conditions, some mounts will draw 230 mA or more when under control, and in other situations, different mounts could draw much less, 150 mA or thereabouts.

Once the heater is "in control", the temperature of the body of the thermistor mount is closely regulated. This regulation minimizes the impact of environmental temperature changes, such as air flow or handling the mount. Any action that will impact heat loss from the mount will have a short-term impact on the DC substituted power. This includes connecting the mount to a splitter or other mount, which will work as a heat sink. Therefore, it is good practice to wait ten minutes after connecting the mount before taking measurements, to allow the gradients to stabilize again.

The old Weinschel 1805B and 1806 meters included a milliammeter for each heater with a "green" zone. I think it's clear from this discussion that being at any particular current level in not an indication that the mount is ready to use. The BEST indication is that the biased thermistors will hold zero within a few microwatts for several minutes. A reasonable indication that the mount is ready to start using the ZERO process is that the mount has been heated for over an hour, that the current has stopped changing, and that at least ten minutes have elapsed since any RF or DC connections have been changed.